Method of forming a multi-layer coating on automobile bodies without a primer bake

ABSTRACT

This invention relates to method for forming a multi-layer coating, comprising sequentially applying a layer of primer coating composition, a layer of base coat composition, and a layer of clear coat composition on an automotive substrate in a wet-on-wet-on-wet manner, and simultaneously curing the applied three layers together in a single baking step. The primer surfacer comprises a film forming binder, a volatile organic liquid carrier, and pigment(s); and the binder contains about (a) 40 to 95% by weight of a caprolactone-modified highly branched acrylic polymer having a hydroxyl and/or carboxyl monomer content, all or part of which has been reacted with caprolactone, of about 1 to 65% by weight and a weight average molecular weight of about 10,000 to 150,000; and (b) 5 to 60% by weight of an aminoplast resin crosslinking agent. The composition is essentially free of crosslinked nonaqueous dispersion resin particles or crosslinked microgel resin particles or both. The resulting cured multi-layered coating has excellent aesthetic appearance, strike-in resistance, chipping resistance, sag resistance, and film build even when formed in a three wet layered application method.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. §119 from U.S.Provisional Application Ser. No. 60/724,511, filed Oct. 7, 2005.

FIELD OF THE INVENTION

The invention concerns a method of forming a multi-layer coating on anautomotive body or part thereof and in particular to a method of formingmulti-layer coating with which a good finished appearance can beobtained by baking the primer, basecoat, and clearcoat layers at thesame time, and also to a primer composition which has excellentresistance to interfacial bleeding with the top coated film and can beused in the forgoing method.

BACKGROUND OF THE INVENTION

Coating systems for automobiles normally comprise a multiplicity ofcoatings applied to a steel substrate. Typically, the steel is treatedwith a rust-proofing phosphate layer, then a cathodic electrocoat primerfor additional corrosion protection is applied. A primer-surfacer (alsoknown as a chip resistant primer, primer, or primer filler) is used nextto smooth the surface for topcoating and also to provide stone chipresistance to the coating system during the normal course of driving.Then a top-coat system is applied, sometimes as a single colored coat,more often now as a basecoat with solid color or flake pigments followedby a transparent protective clear coat, to protect and preserve theattractive aesthetic qualities of the finish on the vehicle even onprolonged exposure to the environment or weathering.

Coating film formation of the basecoat and the clearcoat is normallyachieved by wet-on-wet application, which is to say that the clearcoatis applied to the basecoat without baking the basecoat prior toclearcoat application (although the basecoat may be flash dried for ashort period of time at room temperature prior to clearcoatapplication), and then subsequently baking the basecoat and clearcoat atthe same time to form a dried and cured finish. In the conventionalmethod for forming the multi-layer coating film, the underlying primersurfacer layer, however, is baked before being topcoated with basecoatand clearcoat. Historically, baked primers have been used not only toprovide a smooth surface on which to apply the topcoat, but also to alsoprevent interfacial bleeding or intermixing with the overlying basecoatand avoid disrupting the appearance of the overall topcoat finish.Resistance to intermixing (sometimes referred to as “strike-in”resistance) is especially important for the appearance of glamourmetallic finishes which are popular nowadays on automobiles and trucks.Any disturbance of the metallic pigment flake orientation in metallicbasecoats after application over the primer-surfacer will detract fromthe metallic effect of the finish. Therefore, care must be taken toensure that the metal pigment flakes are not disturbed after painting.

In recent years, it has also been strongly desired to reduce theenvironmental load or impact of automotive assembly plants by reducingVOC (volatile organic compounds) emissions and CO₂ (carbon dioxide)emissions generated from operating painting booths and baking ovens.This has led to use of lower solvent content in the paint and thedevelopment of three-layer wet paint systems which make it possible toapply a primer surfacer, basecoat and clearcoat wet-on-wet continuouslybefore they are cured all at once in a single bake. With this simplifiedapplication process, it is possible to eliminate the separate primerpainting booth and primer oven, which also results in substantial costsavings to the automobile manufacturers. The technical hurdles of thisprocess simplification, however, have been significant. For instance,interfacial bleeding and aesthetic appearance, as well as filmproperties such as chip resistance are still significant concerns.

Attempts have been made to address the forgoing problems by modifyingthe formulation of the primer coating material. U.S. Pat. No. 6,863,929of Watanabe et al. describes a method for forming a multilayerautomotive coating film using a three layer wet paint process (alsoreferred to as a “3 wet” or a “3-coat-1-bake” process) wherein astandard polyester-melamine primer coating is formulated to also containacrylic polymer particles, namely in the form of internally crosslinkednonaqueous dispersion (NAD) polymers or internally crosslinked microgelparticles. These particles are intended to raise the viscosity andsolubility parameter between the primer surfacer and the base coating toprevent intermixing at the interface between the coated layers. However,use of such particle-filled systems also suffers from some drawbacks.

For example, the microparticles also tend to create voids in the surfaceof the wet primer where the basecoat can still flow in and intermix,resulting in defects in the aesthetic appearance such as loss ofsmoothness, gloss, head on brightness, and/or metallic effect. Saggingof these coatings, especially on vertical panels, such as doors,fenders, rocker panels, etc, is also a problem. These particle-filledsystems are also not able to maintain dry film builds at normalcommercial levels. Film builds must therefore be reduced to allow theNAD or microgel particle to migrate to the interface. Yet, thin filmsare an impediment as they tend to subject the underlyingcorrosion-protective electrocoated primer layer to excessive UV lighttransmission and deterioration. Thin films or thin film regions are alsoinadequate for mechanical properties and visual appearance of theoverall finish.

Therefore, there is still a need to find a more effective way to preventthe inter-mixing of the primer surfacer and basecoat and clearcoatlayers when applied in a wet on wet on wet (i.e., a 3 wet) manner andmake it possible to eliminate the primer baking process and reduce theenvironmental impact of the coating system, while also maintaining filmbuilds, the overall appearance such as high gloss and distinctness ofimage and film properties of the coating system.

The present invention has the aforementioned desirable characteristics.

SUMMARY OF THE INVENTION

Disclosed herein is a method for forming a multi-layer coatingcomprising sequentially applying a layer of a primer coatingcomposition, a layer of a base coating composition and a layer of aclear coating composition on a substrate; and simultaneously curing theapplied three layers by baking, wherein the primer coating compositioncomprises: a film forming binder and an organic liquid carrier, andoptionally pigment(s) in a pigment to binder weight ratio of about1:100-150:100; and the binder contains about:

(a) 40 to 95% by weight, based on the weight of the binder, of acaprolactone modified branched acrylic polymer having a hydroxyl and/orcarboxyl monomer content, all or part of which has been reacted with acyclic lactone, of about 1 to 65% by weight and a weight averagemolecular weight of about 10,000 to 150,000; and

(b) 5 to 60% by weight, based on the weight of the binder of acrosslinking agent selected from the group consisting of an aminoplastresin, a blocked polyisocyanate resin, or a mixture thereof.

Also disclosed is a multi-layer coating, comprising: a primer surfacer;a pigmented basecoat; and a clearcoat applied over the basecoat, whereinthe primer surfacer is the multi-layer coating prepared by the abovemethod.

A further disclosure is a primer coating composition comprising a filmforming binder and an organic liquid carrier, and optionally pigment(s)in a pigment to binder weight ratio of about 1:100-150:100; and thebinder contains about:

(a) 40 to 95% by weight, based on the weight of the binder, of acaprolactone modified branched acrylic polymer having a hydroxyl and/orcarboxyl monomer content, all or part of which has been reacted with acyclic lactone, of about 1 to 65% by weight and a weight averagemolecular weight of about 10,000 to 150,000; and

(b) 5 to 60% by weight, based on the weight of the binder of acrosslinking agent selected from the group consisting of an aminoplastresin, a blocked polyisocyanate resin, or a mixture thereof.

Also disclosed herein is a substrate coated with a dried and cured layerof the above composition of claim.

Yet another disclosure is a method for obtaining normal film builds onan automotive substrate using a 3-layer wet paint system without aprimer bake, which method comprises,

(a) applying a layer of the primer composition of claim 8 to asubstrate;

(b) applying a layer of a base coating composition wet-on-wet over saidlayer of primer composition;

(c) applying a layer of a clearcoat composition wet-on-wet over saidlayer of base coating composition;

(d) curing the applied three weight layers together in a single bake.

The present invention now provides a method and a primer coatingcomposition for forming a multi-layered coating, which is capable ofcontrolling intermixing of adjacent paint layers, and interfacialbleeding, and inversion at the interface between each coated layer whena primer coating, a base coating, and a clear coating are appliedsequentially over each other in a wet-on-wet (i.e., wet-on-wet-on-wet)manner on a substrate before being baked together, while still meetingtoday's performance requirements such as good appearance, chipperformance, and film builds.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a three-layer wet paint applicationprocess in accordance with the present invention.

FIG. 2 is a schematic diagram of a conventional automotive coatingprocess that requires a separate primer spray booth and primer bakingprocess.

FIG. 3 is a graph showing the appearance of a horizontally baked panelcoated by the process of this invention.

FIG. 4 is a graph showing the appearance of a vertically baked panelcoated by the process of this invention.

FIG. 5 is a graph showing flop (metallic effect) of a horizontally bakedpanel coated by the process of the invention.

FIGS. 6A, 6B and 6C are micrographs at 100× magnification showingcross-sectional views of panels coated by the process of this invention,in comparison to a conventional primer baking process and also incomparison to a three wet process similar to that of the invention butusing a commercial baking primer.

DETAILED DESCRIPTION OF THE INVENTION

More particularly, the present invention provides a method for forming amulti-layer coating of automotive quality and appearance on a substratewithout the need for a primer bake and the need to reduce film buildsbelow normal commercial levels, comprising the steps of sequentiallyapplying a primer coating composition, a basecoating composition, and aclear coating composition in a wet-on-wet manner on an automotivesubstrate, such as over the entire vehicle body or part thereof,preferably on which an electrodeposition coated film has been formed,and simultaneously curing the applied three layers by baking, whereinthe primer coating composition comprises: a film-forming binder and anorganic liquid carrier, and optionally, but preferably, pigment(s); andthe binder contains about:

(a) 40 to 95% by weight, based on the weight of the binder, of asubstantially non-gelled, caprolactone-modified branched acrylic polymeras the interfacial control polymer having a hydroxyl and/or carboxylmonomer content of about 1 to 65% by weight, all or part of which isreacted with caprolactone, and optionally containing otherfunctionalized monomers for improved crosslinking, and having a weightaverage molecular weight of about 10,000 to 150,000; and

(b) 5 to 60% by weight, based on the weight of the binder of acrosslinking agent selected from the group consisting of aminoplastresin, blocked polyisocyanates, or mixtures thereof; and

wherein the composition is, preferably, essentially free to totally freeof crosslinked NADs or crosslinked microgel resin particles or both.

The composition is also preferably formulated as a low VOC, high solidscomposition having a total solids content of about 40-70% by weight atthe time of application.

The invention is based on the discovery that use of certain relativelyhigh molecular weight caprolactone-modified branched acrylic polymers inthe primer composition, which serve as interfacial control polymers,enables the composition to effectively prevent intermixing of the primerand basecoating layers when the basecoat which follows is applied overthe primer in a wet on wet manner, while still providing an aestheticappearance and film properties such as chip resistance and solidscontent and film builds equal to that of conventional baked primers.

The invention also provides a high solids solvent-borne primer coatingcomposition comprising the aforesaid ingredients (a) to (b), for use inthe aforesaid method for forming a multi-layer coating. The behavior ofthe primer defined above allows for high film builds, excellentappearance such as high gloss, distinctness of image, and desired visual(such as metallic or pearlescent) effect, and excellent chip resistance(a minimum rating of 5 using SAE J-400), despite the absence of a primerbake.

Also included within the scope of this invention is a substrate, such asa vehicle body or part thereof, coated by the method and with thecoating composition disclosed herein.

The invention is especially useful for finishing the entire exteriorbody of automobiles and trucks and parts thereof.

In this disclosure, a number of terms and abbreviations are used. Thefollowing definitions are provided.

“Wet-on-wet” means that an overlying coat is applied to an underlyingcoat without curing (i.e., baking) or completely drying the underlyingcoat.

“Wet-on-wet on-wet”, also used interchangeably herein with “three layerwet”, “3 wet”, and “3-coat-1-bake”, means that the primer layer,basecoat layer, and clearcoat layer are applied successively in awet-on-wet manner.

“Essentially free” with respect to the primer coating shall mean thatthe primer coating composition contains less than 1% by weight,preferably zero percent by weight, of the specified component, based onthe total weight of the composition.

“High solids composition” means a low solvent solvent-borne liquidcoating composition having a total solids content at time of applicationof at least 40 percent, preferably in the range of from 40-70 percent,in weight percentages based on the total weight of the composition. Itshould be understood that “total solids” refers to the total amount ofnon-volatile components in the composition even though some of thecomponents may be non-volatile liquids rather than solids at roomtemperature.

“Caprolactone-modified acrylic polymer” means a polyester-extendedacrylic polymer that has been extended with caprolactone such asepsilon-caprolactone. The polyester chain extension may be at a chainend or it may at any other point along the acrylic backbone. Of course,one skilled in the art would understand that other cyclic lactones canbe used instead of caprolactone and is intended to be included in thisdefinition, unless otherwise indicated.

“Substantially non-gelled” or “non-gelled” refers to reaction productsthat are substantially free of crosslinking and that have a measurableintrinsic viscosity when dissolved in a suitable solvent for thepolymer. As is well known in the art, the intrinsic viscosity of apolymer is determined by plotting the reduced viscosity versus theconcentration and extrapolating to zero concentration. A gelled reactionproduct is essentially of infinite molecular weight and will often havean intrinsic viscosity that is too high to measure.

“Low VOC composition” means a coating composition that has less thanabout 0.6 kilogram of organic solvent per liter (5 pounds per gallon) ofthe composition, preferably in the range of less than about 0.42kilogram of organic solvent per liter (3.5pounds per gallon), asdetermined under the procedure provided in ASTM D3960.

Primer Coated Layer

In the present method for forming a multi-layer coating, a novel primersurfacer coating composition having the ability to prevent inter-mixingof the top coated layer when applied wet-on-wet thereover is employed.This primer surfacer, primer filler, or chip resistant primer,hereinafter “primer”, can be used in the three layer wet paint methoddescribed herein without sacrificing good finished appearance and goodchip performance and good film builds.

The solvent-borne primer composition is not only useful in a wet-on-wetapplication process, it can be formulated to have a low VOC content(volatile organic content), can be formulated into a gray or coloredcomposition that is easy to hide, forms finishes that are hard but stillflexible, has excellent adhesion to a variety of substrates such as coldrolled steel, phosphatized steel, phosphatized steel primed with anelectrocoat primer applied by electrocoating, plastic substrates whichmay be preprimed or unprimed such as polyester reinforced fiber glass,reaction injection molded urethanes, partially crystalline polyamidesand other plastic substrates and provides a surface to whichconventional topcoats will adhere.

The primer composition is particularly useful on the aforementionedsubstrates since it can be used as a surfacer or filler to coverimperfections in surfaces of primed metal and plastic substrates. Forexample, electrocoating of metal substrates with a primer often resultsin a finish that has small imperfections and this composition can beapplied to form a smooth, glossy finish that is free from imperfections.Also, plastic substrates such as SMC (sheet molding compound) which is apolyester reinforced with fiber glass contains many surfaceimperfections and must be coated with a surfacer.

The novel primer composition of this invention generally contains a filmforming binder and a volatile organic liquid carrier, which usually is asolvent for the binder. It is generally desired that the composition beformulated as a low VOC composition. Accordingly, for low VOCcompositions, the primer composition typically has a film forming bindercontent of about 40-85% by weight and correspondingly about 15-60% byweight of volatile organic liquid carrier. Generally, the compositionalso contains pigments in a pigment to binder weight ratio of about1:100-150:100.

As indicated above, the film-forming portion of the primer compositionof this invention is referred to as the “binder” or “binder solids”. Thebinder generally includes all the film-forming components thatcontribute to the solid organic portion of the cured composition.Generally, catalysts, pigments, and non-polymeric chemical additivessuch as stabilizers described hereinafter are not considered part of thebinder solids. Non-binder solids other than pigments usually do notamount to more than about 5-15% by weight of the composition. In thisdisclosure, the term “binder” or “binder solids” refers to thefilm-forming caprolactone-modified branched acrylic polymer, themelamine or polyisocyanate crosslinking agent, and all other optionalfilm-forming components, as are further described hereinbelow.

In a preferred embodiment, the binder or film forming constituent usedin the composition generally comprises about 40-95% by weight of theaforesaid substantially non-gelled, caprolactone-modified branchedacrylic polymer, which is sometimes referred to herein as “highlybranched” or “hyper branched” or “branched” acrylic polymer, and about5-45% by weight of an aminoplast resin cross-linking agent. It should beunderstood that a blocked polyisocyanate crosslinking agent can be usedto replace some portion or all of the aminoplast, if desired. Blockedpolyisocyanates are however known to increase the overall cost of thecomposition and therefore are less desirable. For most uses, thecomposition typically contains about 65-75% by weight ofcaprolactone-modified branched acrylic polymer and 25-35% by weight ofaminoplast resin cross-linking agent.

In general, the substantially non-gelled caprolactone-modified branchedacrylic polymer has a Mw (weight average molecular weight) of about10,000 to 150,000, more preferably in the range from about 30,000 to120,000, a hydroxyl and/or carboxyl monomer content of about 1 to 65% byweight; and caprolactone mols of about 0.25 to 6 per mole to hydroxyland/or carboxyl in the mixture being polymerized, preferably about 2moles of caprolactone per mole of hydroxyl. In a preferred embodiment,the polymer is essentially free of carboxyl functional groups.

All molecular weights described herein are determined by gel permeationchromatography using polystyrene as the standard.

While not wishing to be bound by theory, the inclusion of the forgoingacrylic polymer is believed to act as an interfacial control polymer andthus prevent intermixing of the wet primer and basecoating layers by (1)decreasing permeability of the primer enough to prevent bleeding, butstill maintaining sufficient low viscosity so as to enable easyapplication such as by spraying, without the need to employ appreciableamount of volatile solvents, and/or by (2) choosing a chemistry,primarily acrylic chemistry, that is preferably immiscible with thelayer that follows which is the basecoat layer.

In order to form the desired branched caprolactone-modified acrylicpolymer, the polymer is preferably composed of caprolactone and at leasttwo types of ethylenically unsaturated monomers, namely 1) at least onemonoacrylic monomer and 2) at least one diacrylic or dimethacrylicmonomer. Optionally the polymer may additionally contain 3) at least onemonomethacrylic monomer, provided that it does not exceed 40% by weightof the total reaction mixture. In a preferred embodiment, the monomermixture contains no more than 30% by weight diacrylic and/ordimethacrylic monomers in total, to minimize gel formation under theabove described reaction conditions.

A portion of the ethylenically unsaturated monomer structures mentionedabove should also contain a hydroxyl and/or carboxyl group or othergroup containing an active hydrogen capable of reacting with thecaprolactone monomer in order to chain extend the polymer with thelactone and also to provide crosslinking functionality to the polymer.Hydroxyl groups are generally preferred. Examples of hydroxyl containingmonoethylenically unsaturated monomers that can be used to introducesuch hydroxyl groups are hydroxyalkyl acrylates and hydroxyalkylmethacrylates such as: 2-hydroxyethyl acrylate, 2-hydroxypropylacrylate, 3-hydroxypropyl acrylate, 4-hydroxybutyl acrylate,2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate,3-hydroxypropyl methacrylate, and 4-hydroxybutyl methacrylate. Anotherexample of a hydroxyl functional (meth)acrylate monomer, which is usefulherein, is one which has already been reacted with caprolactone such asTONE M-100®, a product of Union Carbide which is the reaction product ofone mole of 2-hydroxyethyl acrylate with 2 moles ofepsilon-caprolactone.

Examples of carboxyl containing monoethylenically unsaturated monomersare: acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaricacid, and crotonic acid. The amount of hydroxyl and/or carboxylfunctionality may vary, depending on the final properties desired. In apreferred embodiment, up to 65%, preferably from 5 to 40%, morepreferably from about 10 to 20%, by weight of the monomer mixturecontains hydroxyl and/or carboxyl functionality to allow the polymer tobe chain extended and have the desired crosslinking functionality, highmolecular weight and intermixing or strike-in resistance but stillsufficiently low viscosity.

Optionally, besides the hydroxyl and/or carboxyl groups mentioned above,the caprolactone modified branched acrylic polymer may containadditional functional groups (up to about 65% by weight functionalmonomers in the monomer mixture) such as amino, carbamate, alkoxy silanesuch as trimethoxy silane, epoxy and the like, to impart additionalcrosslinking functionality to the polymer and enhance the integrity ofthe cured coating. Of course, the amount of functional groups may vary,depending on the final properties desired. These functional groups canbe introduced by employing a functional monomer containing the desiredgroup in the polymerization process or by post-reaction of a polymer ofthe invention to introduce the desired additional functionality, as willbe apparent to those skilled in the art.

Examples of such functional monomers are silane-containing monomers,particularly alkoxy silanes such as gamma-acryloxypropyltrimethoxysilane, gamma-methacryloxypropyl trimethoxysilane (Silquest®A-174 from Crompton), and gamma-methacryloxypropyltris(2-methoxyethoxy)silane. Examples of useful amine-containing monomers areN,N-dimethylaminoethyl methacrylate (tertiary amine), N,N-dimethylaminoethyl acrylate (tertiary amine), N-t-butylaminoethylmethacrylate (secondary amine), N-t-butylaminoethyl acrylate (secondaryamine), 2-aminoethyl methacrylate hydrochloride (primary amine), and thelike. Examples of useful epoxy containing monomers are glycidylmethacrylate and glycidyl acrylate and any acrylic monomer with ahydroxyl group that can be reacted with epichlorohydrin to produce theepoxy group containing monomers. Examples of useful carbamate containingmonomers include adducts of aliphatic alcohols with 2-isocyanatoethylmethacrylate. Methods for preparation if carbamate functionalizedacrylics are well known in the art and described, for example, in EP 0594 142 B1 and EP 0 719 795 B1, the disclosures of which are herebyincorporated by reference herein.

Typically, the remainder of the ethylenically unsaturated monomers inthe monomer mix will be non-functional monomers containing no carboxylicacid groups, hydroxyl groups or other reactive or crosslinkablefunctional groups.

Examples of non-functional monoacrylic and methacrylic monomers arealkyl acrylates and methacrylates such as: methyl acrylate, ethylacrylate, propyl acrylate, isopropyl acrylate, butyl acrylate, isobutylacrylate, hexyl acrylate, 2-ethylhexyl acrylate, nonyl acrylate, laurylacrylate, stearyl acrylate, cyclohexyl acrylate, isodecyl acrylate,propyl acrylate, phenyl acrylate, isobornyl acrylate, methylmethacrylate, ethyl methacrylate, butyl methacrylate, t-butylmethacrylate, hexyl methacrylate, 2-ethylhexyl methacrylate, nonylmethacrylate, lauryl methacrylate, stearyl methacrylate, cyclohexylmethacrylate, isodecyl methacrylate, propyl methacrylate, phenylmethacrylate, isobornyl methacrylate and the like, or other constituentssuch as styrene or substituted styrene, such as methyl styrene,acrylonitrile, and methacrylonitrile, acryamide, and methacrylamide, andthe like.

Examples of diacrylic and methacrylic monomers for use as a co-monomerin the monomer mix to impart branching are diesters of acrylic andmethacrylic acids, such as: ethylene glycol dimethacrylate anddiacrylate, diethyleneglycol dimethacrylate and diacrylate,triethyleneglycol dimethacrylate and diacrylate, 1,3-propanedioldimethacrylate and diacrylate, 1,4-butanediol diacrylate, 1,6-hexanedioldiacrylate, 2,2-dimethylpropanediol diacrylate, tripropylene glycoldimethacrylate and diacrylate, 1,3-butylene glycol dimethacrylate anddiacrylate.

Urethane diacrylates and dimethacrylates can also be used, since theyimpart in coating applications, increased flexibility to the curedcoating layer and reduced brittleness, when used in the correctproportion with the other essential ingredients in coating applications.The urethane monomers can be produced by any of the methods known tothose skilled in the art. Two typical methods are 1) reacting adiisocyanate with a hydroxy-containing acrylate or hydroxy-containingmethacrylate, such as 2-hydroxyethyl acrylate or 2-hydroxyethylmethacrylate; and 2) reacting an isocyanatoalkyl acrylate or anisocyanatomethacrylate with a suitable diol. Some of the diethylenicallyunsaturated monomers, as can be seen from the list of these monomersabove, may also contain a functional group, such as any of those listedabove, to impart crosslinking functionality to the polymer.

As mentioned above, the polymer may also contain some monomethacrylicmonomers. However, when such monomers are employed in the free-radicalpolymerization reaction, it is desired that the total amount ofmonomethacrylic monomers in the monomer mixture should not exceedapproximately 40% by weight. Higher amounts can be used but at amountsexceeding 40% by weight, such monomers begin to interfere with thebranching mechanism (or so-called “backbiting” described hereafter) andthus result in a polymer of a lower degree of branching, as demonstratedby a sharp rise in viscosity, which is undesirable. The products formedat such concentrations are quite viscous and difficult to handle.

In addition, among the mono acrylic and methacrylic monomers listedabove, it is generally desired to include (up to about 70% by weight ofthe monomer mixture) of at least one bulky monomer selected from thegroup consisting of isobornyl (meth)acrylate, butyl (meth)acrylates (allisomers), ethyl hexyl(meth)acrylate (all isomers),cyclohexyl(meth)acrylate, or mixture of these monomers, preferably toincrease the intermixing or strike-in resistance of the coatingcomposition with overlying coating layers applied wet-on-wet thereover.

Also, since the intended end use of the branched acrylic polymer productis in a high solids primer coating composition, the amount of diacrylicor dimethacrylic monomer(s) will generally not exceed 30% by weight ofthe total monomer mixture to avoid gelation, although this may varydepending on the particular diacrylic or dimethacrylic monomersemployed, as well as the composition of the monomer mixture.

In a preferred embodiment, the branched acrylic polymer is composed ofpolymerized monomers of a first acrylate monomer which is eitherisobornyl acrylate, butyl acrylate (all isomers), ethyl hexyl acrylate(all isomers), or cyclohexyl acrylate, or mixture of these monomers, anda second methacrylate or acrylate monomer which is either a hydroxyalkyl methacrylate or acrylate that has 1-4 carbon atoms in the alkylgroup, or a carboxyl containing acrylic or methacrylic monomer, ormixtures of these monomers, a diacrylate or dimethacrylate monomer or amixture of these monomers, and polymerized caprolactone grafted thereto,wherein all or part of the hydroxyl and/or carboxyl groups are reactedwith caprolactone to form the lactone graft chain on the branchedacrylic polymer either prior to, during, or after free-radicalpolymerization.

One especially preferred branched acrylic polymer contains about 40-98%by weight of the first acrylate, 1-30% of the second acrylate ormethacrylate, and 1-30% by weight of the diacrylate or dimethacrylate.Of course, the total percentage of monomers in the polymer equal 100%and therefore if an amount equal to or approaching the maximum of oneparticular monomer is employed, then the relative amounts of theremaining monomers must be reduced accordingly.

One particularly preferred branched acrylic polymer contains thefollowing constituents in the above percentage ranges: isobornylacrylate, hydroxy ethyl methacrylate, and 1,6-hexanediol diacrylate,wherein the hydroxyl groups are reacted with caprolactone, preferablyepsilon-caprolactone, to form the lactone graft on the branched acrylicpolymer.

Of course other cyclic lactones can also be used, as will be apparent tothose skilled in the art. Besides episilon-caprolatcone, some of thesuitable lactones include gamma-caprolactone; gamma-butyrolactone;gamma-valerolactone; delta-valerolactone; gamma-butyrolactone; andlactones of the corresponding hydroxy carboxylic acids, such as,glycolic acid; lactic acid; 3-hydroxycarboxylic acids, e.g.,3-hydroxypropionic acid, 3-hydroxybutyric acid, 3-hydroxyvaleric acid,and hydroxypyvalic acid. However, the most preferred of these isepsilon-caprolactone.

The caprolactone-modified branched acrylic polymer described above canbe prepared by a variety of solution polymerization methods in which themonomers are blended with a liquid reaction medium, a free radicalpolymerization initiator, optionally caprolactone, optionallycaprolactone modified monomer, optionally a polymerization catalyst forthe caprolactone, and optionally a chain transfer agent, and heated to arelatively high temperature of typically at least 130° C., preferably atleast 150° C., more preferably at least. 160° C., for a sufficient time,as will be apparent to those skilled in the art, typically for about 2to 8 hours to form a substantially non-gelled branched polymer. Ingeneral, at temperatures below 130° C., the amount of internalcrosslinking increases and also the relative amount of by-productsincreases. Furthermore, at too low a reaction temperature, the viscosityof the reaction mixture rapidly increases to a point where the reactionmixture is too viscous to be stirred and the reaction is then difficultto control and must be terminated. When the caprolactone is not includedin this process, it is added to the preformed branched acrylic polymeralong with a polymerization catalyst for the caprolactone and heated to75° C. to 165° C. for a sufficient time, as will be apparent to thoseskilled in the art, typically for 2 to 8 hours to form acaprolactone-modified branched acrylic polymer.

As indicated above, the free radical polymerization portion of theprocess used herein to form the branched acrylic polymer backbone may becarried out utilizing conventional techniques, such as by heating themonomers in the presence of initiators and/or catalysts and varyingsolvents, with the proviso that the reaction temperature duringpolymerization must be high enough (i.e., generally above 130° C.) toinduce branching without causing the polymer to gel.

While not wishing to be limited by any particular mechanism, it isbelieved that the high temperature free-radical polymerization processused herein involves so-called “backbiting” which prevents gelation ofthe monomer mixture. In the polymerization process described, it isbelieved that abstraction of a methine backbone hydrogen occurs to givea tertiary radical which leads to formation of a branching point andultimately a branched polymer through subsequent monomer addition.Abstraction of the hydrogen from the backbone is believed to occur byintramolecular chain transfer, or so-called backbiting, which bestaccounts for the observed branching, as opposed to formation of a gelledpolymer, as would be expected to normally occur in classical freeradical polymerization that utilizes greater than insignificant amountsof diacrylate or dimethacrylate monomers. Such backbiting reactions inhigh temperature acrylate polymerization are described more fully inPeck and Grady, Polym. Preprints, 2002, 43(2), 154, hereby incorporatedby reference.

In the present invention, it has been unexpectedly observed that even inthe presence of diacrylic or dimethacrylic monomers, higher reactiontemperatures favor this backbiting, with little or no gelled polymerbeing formed. It was previously thought that the presence of largeamounts of diacrylic or dimethacrylic monomers in the reaction mixturewould cause the reaction mixture to gel. The process disclosed thereforeemploys rather high reaction temperatures to increase the incidence ofbackbone hydrogen abstraction and increase the incidence of branching.Increasing the number of branching points on a polymer chain leads tolower viscosity. It is well known that the inherent viscosity ofbranched polymers is lower than for corresponding linear polymers ofequal molecular weight, which allows the branched acrylic polymer soformed to be used in a high solids coating with viscosity low enough forpractical application such as by spraying.

The free radical polymerization portion of the process that is used toform the acrylic polymer backbone and branched structure is preferablycarried out in the presence of a free radical polymerization initiator,typically, tertiary butyl perbenzoate, tertiary butyl peroctoate, cumenehydroperoxide, benzoyl peroxide, di-tertiary butylperoxide, di-cumeneperoxide, methyl ethyl ketone peroxide or similar peroxygen compounds,or an azo compound such as azobisisobutyronitrile is employed. Theamount of free radical polymerization initiator can be varied dependingupon the desired molecular weight but about 0.05-8% by weight based onthe weight of total polymerizable monomer is typical. A preferred rangeis from 0.05 to 4 percent by weight. A mixture of two or more initiatorsmay be used.

A solvent is not essential but is preferably used as the liquid reactionmedium. The solvent can be used at from 0 to about 75% of the totalreaction mixture preferably from 30 to 55 percent by weight of thereaction mixture. Any of the conventional polymerization solvents may beutilized in the present high temperature process to prepare the branchedacrylic polymers. The higher boiling solvents are preferred due to theirlow vapor pressure at the high temperature required to induce branching.In general, solvents having a boiling point above 100° C., especially150° C. are most preferred. Examples of such higher boiling solventsinclude esters and mixed ethers and esters, Cellosolve (registeredtrademark of the Union Carbide Corporation), butyl Cellosolve,Cellosolve acetate, the Carbitols (registered trademark of the UnionCarbide Corporation), the (poly) alkylene glycol dialkyl ethers and thelike. Any solvent is acceptable as long as the functionality of thesolvent does not interfere with the monomer functionality. The reactionmay also be run under pressure so that the boiling point of a lowboiling solvent can be increased to temperatures desired to produce thepolymers of the present invention.

Further, various hydrocarbon fractions may be utilized with the mostpreferred being Solvesso 150 or Solvesso 100 (a registered trademark ofthe Exxon Mobil Oil Company). Aromatic solvents can also be employed,for example, toluene, xylene, cumene, and ethyl benzene. Special care isexercised when functional solvents are desired. Acid, alcohol and aminefunctional solvents have the potential of reacting with caprolactone,and therefore should not be introduced until the caprolactone has beenreacted with the desired site on the branched acrylic polymer.

Once the monomers capable of reacting with a cyclic lactone or monomerswhich have been pre-reacted with lactone are included in the reactionmixture, several different processing methods can be used to chainextend the branched acrylic polymer with the cyclic lactone and preparethe final caprolactone modified branched acrylic polymers. The maindifferences involve the specific point where the lactone, preferablycaprolactone, is introduced into the reaction process.

One method useful in the present invention is to pre-react the desiredlactone with the carboxyl or hydroxyl functional ethylenicallyunsaturated monomer in the presence of a suitable catalyst to form a newlactone extended monomer with an ethylenically unsaturated (preferablyacrylic or methacrylic) double bond and a pendant hydroxyl or carboxylgroup. The molar ratio of lactone to ethylenically unsaturated carboxylor hydroxyl monomer can range from about 0.1 to 20 moles, preferably0.25 to 6 moles, most preferably 1 to 3. A typical example of such amonomer is TONE M-100®, a product of Union Carbide which is a reactionproduct of one mole of 2-hydroxyethyl acrylate with 2 moles ofepsilon-caprolactone.

In a second method, the lactone is charged to the reactor along with theorganic solvents. These materials are heated to reaction temperature andthe ethylenically unsaturated monomers are added along with a freeradical catalyst and reacted in the presence of the solvent and thelactone. A catalyst for the lactone polymerization may be addedconcurrently with the acrylic monomers or may be added prior to theaddition of these monomers. The temperature is held for a sufficienttime, as will be apparent to those skilled in the art, to form thedesired chain extended branched acrylic polymer.

In a third method, the branched acrylic polymer is first formed via ahigh temperature polymerization process. When this process is complete,the desired lactone is then added along with a catalyst for the lactonepolymerization and the desired product is formed.

In all cases, the molar ratio of lactone to ethylenically unsaturatedcarboxyl or hydroxyl monomer added to the reaction mixture can vary. Themolar ratio typically ranges from about 0.1 to 20, more preferably fromabout 0.25 to 6. One skilled in the art would be able to vary the amountof polymerization catalyst, the reaction temperature, and otherconditions to affect the lactone polymerization.

In addition to the free radical polymerization catalyst, thepolymerization medium could include a polymerization catalyst whencaprolactone is used in the composition.

Typically this caprolactone catalyst may be an alkali or alkaline earthmetal alkoxide, e.g. sodium or calcium methoxide; aluminum isopropoxide,organotin compounds, e.g., dibutyl tin dilaurate, dibutyl tin diacetate,stannous octoate, and dibutyl tin oxide tetraalkyl titanates, titaniumchelates and acylates, lead salts and lead oxides, zinc borate, antimonyoxide, stannous octoate, organic acids, inorganic acids such assulfuric, hydrochloric, and phosphoric, and Lewis acids such as borontrifluoride. The preferred catalyst is dibutyl tin dilaurate.

In any of the processes described above, polymerization is preferablycontinued until the resulting film-forming branched acrylic polymer hasthe desired molecular weight and requisite branching and crosslinkingfunctionality and desired intermixing and strike-in resistance but stillsufficiently low viscosity for use in the primer coating composition ofthe present invention.

In addition to the above film-forming branched acrylic resin component,the primer composition also contains, as part of the film-formingbinder, a crosslinking agent. The crosslinking agent used in thecomposition is an aminoplast resin or blocked polyisocyanate resin ormixture of the two. Aminoplasts resins such as melamine formaldehydecondensates are generally preferred. In general, aminoplast resins arealdehyde condensation products of melamine, urea, benzoguanamine, or asimilar compound. Usually, the aldehyde employed is formaldehyde,although useful products can be made from other aldehydes, such asacetaldehyde, crotonaldehyde, acrolein, benzaldehyde, furfural, andothers. Condensation products of melamine or urea are the most commonand are preferred, but products of other amines and amides in which atleast one amine group is present can also be employed.

Of the melamine condensates, monomeric or polymeric melamineformaldehyde condensate resins that are partially or fully alkylated aregenerally preferred. These preferred resins are organic solvent-solubleand are commercially available under the tradename Cymel® from CytecIndustries, Inc., West Patterson, N.J. One preferred crosslinking agentis a methylated and butylated or isobutylated melamine formaldehyderesin that has a degree of polymerization of about 1-3. Generally, thismelamine formaldehyde resin contains about 50% butylated groups orisobutylated groups and 50% methylated groups. Another preferredmelamine, for a good balance of properties is, a fully butylated resinknown as Cymel 1156®.

Other possible crosslinking agents, of course, can also be used, such asurea formaldehyde, benzoquanamine formaldehyde and blocked or unblockedpolyisocyanates or compatible mixtures of any of the forgoingcrosslinkers.

For instance, the aminoplast crosslinking agent(s) described above canbe substituted for or optionally combined with any of the conventionalblocked polyisocyanate crosslinking agents for enhanced film properties.Typical blocking agents are alcohols, ketimines, oximes, pyrazoles andthe like.

Typical examples of polyisocyanates are isocyanate compounds having 2 to4 isocyanate groups per molecule, such as 1,6-hexamethylenediisocyanate, isophorone diisocyanate, 2,4-toluene diisocyanate,diphenylmethane-4,4′-diisocyanate,dicyclohexylmethane-4,4′-diisocyanate, tetramethylxylidene diisocyanate,and the like. Polyisocyanates having isocyanurate structural units canalso be used such as the isocyanurate of hexamethylene diisocyanatewhich is available under the tradename Desmodur N-3390® from BayerCorporation of Pittsburgh, Pa., the isocynaurate of isophoronediisocyanate (isocyanurate) which is available under the tradenameDesmodur Z4470® from Bayer Corporation and the like.

Polyisocyanate functional adducts can also be used that are formed fromany of the forgoing organic polyisocyanate and a polyol. Polyols such astrimethylol alkanes like trimethylol propane or ethane can be used. Oneuseful adduct is the reaction product of tetramethylxylidenediisocyanate and trimtheylol propane and is available under thetradename of Cythane 3160®. When the crosslinkable resin of the presentinvention is used in exterior coatings, the use of an aliphatic orcycloaliphatic isocyanate is preferable to the use of an aromaticisocyanate, from the viewpoint of weatherability and yellowingresistance. An example of a suitable blocked isocyanate that can be usedin the present system is a pyrazole blocked polyisocyanate of1,6-hexamethylene diisocyanate which is available from BayerCorporation.

Optionally, in addition to the above film-forming binder constituents,the composition may also contain, as part of the film-forming binder,other film-forming binder resins and/or crosslinking resins, such asacrylic resins, acrylourethane resins, alkyd resins, epoxy resins,polyester resins, polyester urethane resins, and the like. However, asindicated above, the composition should be totally free or essentiallyfree of crosslinked microgel resin particles based on, for example,acrylic microgels, and crosslinked NAD resin particles, based on, forinstance, acrylic NADs, as part of the film-forming binder. If theoverlying basecoating layer is formed from a polyester based coatingcomposition (e.g., a standard polyester-melamine base coating), it isgenerally desired that the primer composition also be free of any of theaforesaid polyester binder resins, in order to further raise thesolubility parameter between the two layers.

Besides the film-forming binder constituents, the coating composition ofthe present invention may also include minor amounts of non-bindersolids. Generally, catalysts, pigments, or chemical additives such asstabilizers are not considered part of the binder solids. Non-bindersolids other than pigments, as indicated above, usually do not amountfor more than about 5-15% by weight of the composition. Such additionaladditives will, of course, depend on the intended use of the coatingcomposition.

For instance, to increase the rate of crosslinking of the composition oncuring, a catalyst can be added to the composition. Generally, about0.1-6% by weight, based on the weight of the binder, of catalyst isused. Typical of such catalyst are blocked acid catalysts. Typicallyuseful blocked acid catalysts are aromatic sulfonic acids blocked withamino methyl propanol or dimethyl oxazoline. Typically useful aromaticsulfonic acids are para toluene sulfonic acid, dodecyl benzene sulfonicacid, decyl benzene sulfonic acid. One preferred catalyst is dodecylbenzene sulfonic acid blocked with amino methyl propanol.

To improve the outdoor weatherability of the composition and protect thecoated substrate from premature degradation, the composition typicallycontains about 0.01-2% by weight, based on the weight of the binder, ofultraviolet light stabilizers which term includes ultraviolet lightabsorbers, screeners and quenchers. Typical ultraviolet lightstabilizers include benzophenones, triazines, triazols, benzoates,hindered amines and blends thereof.

Typical pigments that can be used in the composition are filler pigmentssuch as talc, china clay, barytes, carbonates, silicates, and colorpigment such as metallic oxides such as titanium dioxide, zinc oxide andiron oxide and carbon black and organic colored pigments and dyes. Theresulting primer composition has a pigment to binder weight ratio ofabout 1:100-150:100.

The pigments can be introduced into the primer composition by firstforming a mill base with an acrylic copolymer dispersant or with anothercompatible polymer or dispersant by conventional techniques such as sandgrinding, ball milling or attritor grinding. The mill base is blendedwith other constituents used in the composition.

In general, a gray color primer prepared by using carbon black andtitanium dioxide as main pigments is typically employed. However,various color pigments may be employed to provide various colors forexample that having a hue similar to that of the colored basecoat layerthat is subsequently applied directly thereover. This is done to enablethe colored basecoat to achieve complete hiding at the lowest possiblefilm build. In addition, it is generally desirable to include smallamounts of talc in the composition to improve the chipping resistance ofthe coating film.

As to the liquid carrier, any of the conventional organic solvents orblends of solvents can be used to form the primer composition providedthat the selection of solvents is such that the polymeric binderconstituents are compatible and give a high quality primer coating. Thefollowing are examples of solvents that can be used to prepare thecomposition: methyl ethyl ketone, methyl amyl ketone, methyl isobutylketone, toluene, xylene, acetone, ethylene glycol monobutyl etheracetate and other esters, ethers, ketones and aliphatic and aromatichydrocarbon solvents that are conventionally used. The proportion ofsolvents is not critical, since they primarily serve as the volatilevehicle to convey the solid material to the substrate to be coated. Thesolvent is preferably employed in an amount to provide a stableconcentrate that can be shipped to assembly plants which is laterreduced with solvent to a suitable spray viscosity for ease ofapplication.

In addition to the above ingredients, the composition may also includeother conventional formulation additives such as toughening agents, andflow control agents, for example, such as Resiflow® S(polybutylacrylate), BYK® 320 and 325 (high molecular weightpolyacrylates). Such additional additives will, of course, depend on thedesired final properties of the coating composition, as will be apparentto those skilled in the art. In addition, conventional rheologicallyactive agents, such as Garamite® clay, fumed silica, urea sag controlagents, and the like can also be used, for enhanced intermixingresistance.

As indicated above, high solids primer compositions are generallypreferred for use in the multi-layer coating process of this invention.The primer coating composition preferably has a total solids content (%non-volatile) of about 40 to 70% by weight at the time of application,and preferably between 50 and 65% by weight, based on the total weightof the coating composition, in order to keep air pollution to a minimumlevel. High solids coatings behave like low solids liquid coatings buthave the additional benefit of lower solvent content and significantlyreduced emissions.

The volatile organic content or VOC level at such solids typicallytranslates to less than about 3.5 pounds of organic solvent per gallonof curable composition, as determined under the procedure provided inASTM D3960. It should be understood however, that additional solvent maybe added, if necessary, at the time of application to adjust the sprayviscosity and control the flow and leveling of the coating and provideother desirable properties, as will be apparent to those skilled in theart.

The primer composition can be applied to a plastic or metal substrate byconventional techniques such as spraying, electrostatic spraying,dipping, brushing, flowcoating and the like.

Base Coated Layer

In the method for forming the multi-layer coating according to thepresent invention, a colored base coating material is employed forforming a base coated layer. The base coated layer forms a top coatedfilm together with a clear coated layer which will be described later.This base coating composition contains a film forming resin, usually acuring agent, a color pigment and optionally an effect pigment, toimpart a special visual effect such as sparkle, pearlescent,luminescent, and/or metallic appearance or an increased depth of colorto the cured coating composition.

Any of the conventionally known basecoat compositions can be used in themethod of the invention. In general, the composition of the basecoat isnot limited by the present invention. The base coating composition maybe a solvent type or a water-borne type.

Examples of film forming resins contained in the base coating materialinclude, but are not limited to, polyester resins, acrylic resins, alkydresins, epoxy resins, urethane resins and the like, and resins may beemployed alone or in combination. The film forming resin can be employedin combination with a curing agent. Examples of the typical curingagents include amino resins such as melamine formaldehyde condensatesand/or blocked isocyanate resins.

An example of a typical high solids solvent borne basecoat, in additionto color pigments, optional aluminum flakes, and UV absorber, comprisesby weight of composition, about 10% microgel for rheology control, 21%melamine formaldehyde resin, 15% branched polyester resin, 5% hydroxyfunctional acrylic resin, 1% dodecyl benzyl sulfonic acid catalyst, and40% solvent to disperse and/or dilute the above mentioned polymers andfacilitate spray application.

Clear Coated Layer

For forming the clear coated layer, a clear coating composition isemployed. The clear coating composition is not particularly restrictedand may be a clear coating material which contains a film forming resin,a curing agent and the like. The clear coating material may be a solventtype, a water-borne type or a powder type.

High solids solvent borne clear coats which have low VOC (volatileorganic content) and meet current pollution regulations are generallypreferred. Typically useful solventborne clearcoats include but are notlimited to 2K (two component) systems of polyol polymers crosslinkedwith isocyanate and 1K systems of acrylic polyol crosslinked withmelamine or 1K acrylosilane systems in combination with polyol andmelamine.

Suitable 1K solvent borne acrylosilane clearcoat systems that can beused in the process of the present invention are disclosed in U.S. Pat.No. 5,162,426, hereby incorporated by reference. Suitable 1K solventborne acrylic/melamine clearcoat systems are disclosed in U.S. Pat. No.4,591,533, hereby incorporated by reference.

Epoxy acid systems can also be used. Such finishes provide automobilesand trucks with a mirror-like exterior finish having an attractiveaesthetic appearance, including high gloss and DOI (distinctness ofimage).

Substrate

The method for forming a coating of the present invention may be appliedto various substrates such as metals, plastics and foamed bodies, andcombinations thereof, preferably to metal surfaces and moldings, andmore preferably to metal products on which cationic electrodepositioncoated film has been formed.

Examples of the metal substrates include iron, copper, aluminum, tin,zinc and the like and alloys containing these metals, such as steel.Specific products include bodies and parts of automobiles such aspassenger cars, trucks, motorcycles and buses.

The metal substrates that are particularly preferred are thosepreliminarily subjected to forming treatment with phosphate salt,chromate salt or the like.

The substrate may have an electrodeposition coated film on the surfacesubjected to forming treatment. The electrodeposition coated film may beformed from an anionic or a cationic electrodeposition coating material.However, a cationic electrodeposition coating material is preferredsince it provides excellent corrosion resistance.

Examples of plastic substrates that can be coated according to themethod of the present invention include polyester reinforced fiberglass,reaction-injection molded urethanes, partially crystalline polyamides,and the like or mixtures thereof, which may be primed or unprimed orotherwise treated as well prior to treating by the coating methoddescribed herein. These plastic substrates are oftentimes used infabricating specific automotive body parts, such as fenders, bumpers,and/or trim parts.

Method for Forming Coating

According to the method for forming a multi-layer coating of the presentinvention, as exemplified in FIG. 1, a layer of primer coatingcomposition 12 is formed on a substrate (automobile body 10 shown inFIG. 1) using the primer coating composition, then a layer of a basecoating composition 14 is formed using the base coating composition anda layer of clear coat composition 16 is formed using the clear coatingcomposition in this order in the wet-on-wet manner.

According to the present invention, when the three coating compositionsdescribed above are applied to automobile bodies, conventional coatingmethods such as spraying, electrostatic spraying, high speed rotationalelectrostatic bells, and the like, can be conducted. The preferredtechniques for applying all three layers of coating compositions are airatomized spraying with or without electrostatic enhancement, and highspeed rotary atomizing electrostatic bells, since these techniques aretypically employed in modern automobile and truck assembly plants.

When the primer coating composition is applied to automotive bodiesaccording to the present invention, any of the above techniques can beused.

The primer coating composition forms a cured layer having a thickness ofusually 0.3 to 2.5 mils (7 to 60 μm), preferably 0.5 to 1.5 mils (12 to36 μm), but the thickness may vary according to the intended use. If thethickness is more than the upper limit, image sharpness may deteriorateor a trouble such as unevenness or sagging may occur at the time ofapplication. If it is less than the lower limit, the electro-primedsubstrate cannot be hidden, and film discontinuity may occur, whichcould expose the lower electrocoat layer to excess UV transmission anddegradation.

On the uncured layer of primer coating composition, a layer of a basecoating composition and a layer of a clear coating composition areapplied in the wet-on-wet manner to form a base coated layer and a clearcoated layer.

A layer of base coating composition may be applied, like the primercoating composition, using air-electrostatic spray coating or a rotaryatomizing electrostatic bell so as to have a dry thickness of 0.4 to 1.2mils (10 to 30 μm).

A layer of clear coating composition is then applied on the base coatedlayer, for the purpose of smoothing roughness or glittering which occursdue to the presence of luster color pigment and for protecting a surfaceof the base coated layer. The clear coating composition may be applied,like the base coating composition, using the rotary atomizingelectrostatic bells.

The clear coated layer is preferably formed so as to have a curedthickness of about 1.0 to 3.0 mils, (25-75 μm).

The multi-layered coated layers obtained as described above are thencured (i.e., baked) simultaneously, as shown in FIG. 1, to form alayered coated film. This is what we call “three-coat-one-bake method.”This method requires no oven for drying the primer coating layer beforebeing base coated (which is required in the conventional process shownin FIG. 2), and is favorable from the economical and the environmentalviewpoint.

The multi-layer coating is then cured in a curing oven at a curingtemperature within the range of 100 to 180° C., preferably 130 to 160°C., so as to obtain a cured multi-layer coating with high crosslinkingdensity. The curing time may vary depending on the curing temperature,however, a curing time of 10 to 30 minutes is adequate when the curingtemperature is 130° C. to 160° C.

According to the process of the present invention, the multi-layeredcoating is formed so as to have a cured thickness of 3 to 5 mils (75 to120 μm). It is important to have an adequate film build in each of thelayers of the present invention, as a low film build will affect theappearance, mechanical properties, and the amount of UV transmittance tothe underlying layers. Too low a film build can allow UV radiation topenetrate to the electrocoated layer. Most electrocoat layers are notformulated with UV absorbers and they tend to be very susceptible to UVdegradation.

The following examples further illustrate the present invention,however, these are not to be construed as limiting the present inventionto their details. All parts and percentages are on a weight basis unlessotherwise indicated. All molecular weights disclosed herein aredetermined by GPC (gel permeation chromatography) using polystyrene asthe standard. Unless otherwise specified, all chemicals and reagents canbe obtained from Aldrich Chemical Company, Milwaukee, Wis.

EXAMPLES

The following branched acrylic copolymers were prepared and then used toform the following three wet primer coating compositions of thisinvention.

Example 1 Preparation of High Mw Caprolactone Modified Branched AcrylicPolymer

A 12-liter flask was equipped with a thermometer, stirrer, additionfunnels, heating mantel, reflux condenser and a means of maintaining anitrogen blanket over the reactants. The flask was held under nitrogenpositive pressure and the following ingredients were employed. Weight(gram) Portion 1 Solvesso 100 ® 1758.6 Portion 2 1,6-Hexanedioldiacrylate 703.4 Isobornyl acrylate 3033.6 Hydoxyethyl methacrylate659.5 t-Butylperoxy acetate 44 Solvesso 100 ® 703.4 Portion 3Epsilon-caprolactone 885.3 Solvesso 100 ® 412.2 Dibutyl tin dilaurate3.0 Total 8203

Portion 1 mixture was charged to the flask and the mixture was agitatedand heated to reflux temperature. While maintaining the batch at reflux,Portion 2 was premixed and fed to the flask over a 5-hour period, andthe reaction mixture was held at reflux temperature throughout thecourse of additions. Reflux was continued for another 60 minutes andPortion 3 was premixed and fed to the flask over a 30-minute period atreflux. After addition was completed the reaction temperature was raisedto 150° C. and held for an additional 3 hours. Then the solution wascooled to room temperature and 20 filled out. The weight solids of theresulting polymer solution was 65.8% and the Gardner-Holdt viscosity(ASTM D1545-98) measured at 25° C. was X.

Weight average molecular weight of the polymer was 37690 andpolydispersity was 11, determined by GPC.

Example 2 Preparation of 3 Wet Primer Containing Polymer Above

A gray colored primer surfacer composition was prepared by mixingtogether the following ingredients in a suitable mixing vessel in ordershown: Components Parts by Weight Carbon Black Pigment Dispersion¹ 0.47White Pigment Dispersion² 19.00 Butyl Acetate³ 2.03 Iso Propanol⁴ 4.00Acid Catalyst Solution⁵ 1.50 Monomeric Melamine Formaldehyde (99.8% NV)⁶8.00 Amorphous Silica Dispersion⁷ 10.00 Hyperbranched Acrylic (65% NV)⁸50.00 Barium Sulfate Pigment Dispersion⁹ 5.00 Total 100.00Table Footnotes¹18% Solids of carbon black pigment dispersed in 19% solids of pigmentdispersion agent in ester solvent.²68% Solids of titanium dioxide pigment dispersed in acrylic resin inester solvent.³Butyl acetate solvent.⁴Isopropanol solvent.⁵48% of Nacure ® XP-221, aromatic sulphonic acid, supplied by KingIndustries, Norwalk, Connecticut.⁶Cymel ® 1168, monomeric melamine formaldehyde resin fully alkylated(50% methyl; 50% isobutyl) supplied by Cytec Industries Inc., WestPatterson, New Jersey.⁷9% Solids of Silica dispersion in acrylic resin solution and aromatichydrocarbon solvent.⁸Hyperbranched Acrylic from Example 1.⁹64% of barium sulfate in acrylic resin solution and aromatichydrocarbon solvent.

The resulting 3 wet primer surfacer composition has a theoretical solidcontent of 61% and spray solids was 58% by weight.

Example 3 and Comparative Examples 1-2 3 Wet Coating Method Using 3 WetPrimer Prepared Above Compared to a Conventional Baked Primer Applied bya Conventional Method and 3 Wet Method

Phosphated steel panels were coated in three different ways: (1) using 3wet coating method with primer prepared above (example 3); (2) using a 3Wet coating method but with a standard baking primer (comparativeexample 1); and (3) using a conventional primer baking process with astandard baking primer (comparative example 2) as a control.

In Example 3, the primer surfacer of Example 2 was applied by sprayingprimer-surfacer onto 3 separate phosphatized steel panels coated with acured cathodic epoxy resin based eletrodeposition primer (Cormax®6 EDfrom DuPont Company, Wilmington, Del.) to get film builds of 12, 29 and49 micron. The primer surfacer layers and all following layers wereapplied to the panels using a 55 serrated bell cup. After primersurfacer application, the panels were allowed to air flash dry at roomtemperature for 3 minutes and this was followed by the application ofPueblo Gold solvent borne basecoat (commercial code 647A01099 from DuPont Company) in two coats with flash off 3 minutes getting finalbasecoat film build 18 micron dry and followed by the application ofacrylosilane clearcoat (Gen® 4 ES from DuPont Company, Wilmington, Del.)flashed dried for 10 minutes and baked for 30 minutes 140° C. onvertical and horizontal position for this study.

The 3 wet primer surfacer composition of this invention was applied asabove in comparison with a commercial Titanium Frost 2 in 1 bakingprimer 708-DN079 from DuPont Company applied using the same 3 wetprocess described above (Comparative Example 1) and also applied using aconventional process baking the primer between basecoat application(Comparative Example 2) and the coated panels were then compared. Theresults are reported in FIGS. 3-6 and in the Table below.

FIGS. 3 and 4 show that the 3 Wet Primer (from Example 2) produces anautomotive quality appearance similar to conventional baked primers madeapplied using the standard primer bake technique (Comparative Example2). The figures also show that standard baked primers are incapable ofrunning on a 3 Wet coating line (Comparative Example 1) at automotivequality. The determination of whether the appearance was of automotivequality, i.e., whether the coating had an aesthetic appearance thatmeets the standard of automotive finishes, was determined bymeasurements taken from a WaveScan DOI instrument from BYK Gardner. Thisinstrument measures the visual appearance of a finish by looking atlonger waves that are indicative of a condition commonly known as orangepeel and looking as well at shorter waves which helps to quantify theso-called “distinctness of image” or DOI. These parameters taken incombination (by WaveScan CF readings) can be used to quantify theoverall visual appearance of a vehicle finish. A minimum of horizontal60 and vertical 50 is desirable for automotive use.

FIG. 5 shows the metallic effect or flop of the finishes tested. Theflop values were calculated from measurements determined by the X-Rite®machine from X-Rite Inc., which measures the brightness property of eachpanel from 15°, 45°, and 110® angles. An average of three readings istaken at each angle and the following formula is used to calculate theflop:Flop=((L15°-L110°)*10/L45°).

Chip resistance and adhesion for the multilayer coatings produced abovewere also tested. The following test procedures were used.

Adhesion—the adhesion of 0 to 5 was determined in accordance with testmethod ASTM D3359—a rating of at least 4B is an acceptable minimum.

Chip Resistance—which measures the ability of a coating system to resistphysical damage from impact of a hard material most commonly stones orgravel which are thrown against the vehicle by the wheels of passingcars, or in the case of rocker panels thrown up against the car by thewheels of the same car—was determined utilizing a gravelometer andfollows the procedure described in test method SAE J-400—a rating of atleast 5 is an acceptable minimum.

The test results are summarized in the Table below. TABLE 1 PhysicalProperties of Panels Using DuPont 708 Line Primer Conventional Processversus 3 Wet Primer and 3 Wet Process Chip Primer Process Adhesion (SAEJ400) Comparative 708DN079 Conventional 5 (No failure) 5A Example 2Example 3 3 Wet 3 Wet 5 (No failure) 5A Primer of Example 2

FIGS. 6A, 6B, and 6C are pictures of cross-sectional views of coatedpanels prepared in Examples 3 and Comparative Examples 1 and 2,respectively, and show the levels of intermixing between the primer andprocess. Clearly, the conventional baked primer cannot be run using a 3wet process application process (Comparative Example 1; FIG. 6B), whilethe primer of this invention (Example 2) when applied using a 3 wetprocess (Example 3; FIG. 6A) produced results similar to that of aconventional baked primer applied using a conventional primer bakingprocess (Comparative Example 2; FIG. 6C).

In summary, the results indicated that an automotive quality appearancecan be obtained using the primer coating composition of this inventionin a three-coat-one-bake (i.e., 3 wet) process.

Various other modifications, alterations, additions or substitutions ofthe components of the processes and compositions of this invention willbe apparent to those skilled in the art without departing from thespirit and scope of this invention. This invention is not limited by theillustrative embodiments set forth herein, but rather is defined by thefollowing claims.

1. A method for forming a multi-layer coating comprising sequentiallyapplying a layer of a primer coating composition, a layer of a basecoating composition and a layer of a clear coating composition on asubstrate; and simultaneously curing the applied three layers by baking,wherein the primer coating composition comprises: a film forming binderand an organic liquid carrier, and optionally pigment(s) in a pigment tobinder weight ratio of about 1:100-150:100; and the binder containsabout: (a) 40 to 95% by weight, based on the weight of the binder, of acaprolactone modified branched acrylic polymer having a hydroxyl and/orcarboxyl monomer content, all or part of which has been reacted with acyclic lactone, of about 1 to 65% by weight and a weight averagemolecular weight of about 10,000 to 150,000; and (b) 5 to 60% by weight,based on the weight of the binder of a crosslinking agent selected fromthe group consisting of an aminoplast resin, a blocked polyisocyanateresin, or a mixture thereof.
 2. The method according to claim 1, whereinthe primer composition is totally free or essentially free of particlesof crosslinked nonaqueous dispersion resins and/or crosslinked microgelresins.
 3. The method according to claim 1, wherein the method isperformed on a pre-primed substrate on which an electrodeposition coatedfilm has been formed.
 4. The method according to claim 1 wherein thesubstrate is a vehicle body or part thereof.
 5. A multi-layer coating,comprising: a primer surfacer a pigmented basecoat; and a clearcoatapplied over the basecoat, wherein the primer surfacer is of thecomposition of claim
 1. 6. The multi-layer coating of claim 5 whereinunderneath the primer surfacer is a electrodeposited primer coating. 7.The multilayer coating of claim 5 wherein the coating is an exteriorfinish for automobiles and trucks.
 8. A primer coating compositioncomprising a film forming binder and an organic liquid carrier, andoptionally pigment(s) in a pigment to binder weight ratio of about1:100-150:100; and the binder contains about: (a) 40 to 95% by weight,based on the weight of the binder, of a caprolactone modified branchedacrylic polymer having a hydroxyl and/or carboxyl monomer content, allor part of which has been reacted with a cyclic lactone, of about 1 to65% by weight and a weight average molecular weight of about 10,000 to150,000; and (b) 5 to 60% by weight, based on the weight of the binderof a crosslinking agent selected from the group consisting of anaminoplast resin, a blocked polyisocyanate resin, or a mixture thereof.9. The primer composition according to claim 8 in which thecaprolactone-modified branched acrylic polymer is composed ofcaprolactone and at least two ethylenically unsaturated monomers atleast one having said hydroxyl and/or carboxyl content and the otherhaving no hydroxyl and/or hydroxyl contents.
 10. The primer compositionaccording to claim 9 in which the caprolactone-modified branched acrylicpolymer is composed of caprolactone and at least one monoacrylic monomerand at least one diacrylic or dimethacrylic monomer, and optionally thepolymer may additionally contain at least one monomethacrylic monomer,provided that it does not exceed 30% by weight of the total reactionmixture.
 11. The primer composition according to claim 10 in which themonomer mixture contains no more than 30% by weight diacrylic and/ordimethacrylic monomers in total.
 12. The primer composition of claim 9in which the caprolactone-modified branched acrylic polymer is composedof caprolactone and polymerized monomers of a first acrylate monomerwhich is either isobornyl acrylate, butyl acrylate (all isomers), ethylhexyl acrylate (all isomers), or cyclohexyl acrylate, or mixture ofthese monomers, and a second methacrylate or acrylate monomer which iseither a hydroxy alkyl methacrylate or acrylate that has 1-4 carbonatoms in the alkyl group, or a carboxyl containing acrylic ormethacrylic monomer, or mixtures of these monomers, a diacrylate ordimethacrylate monomer or a mixture of these monomers, and polymerizedcaprolactone grafted thereto.
 13. The primer composition according toclaim 8 in which the aminoplast resin is a partially or fully alkylatedmonomeric or polymeric melamine formaldehyde condensate.
 14. The primercomposition of claim 8 containing in addition about 0.1-6% by weight,based on the weight of the binder, of a blocked acid catalyst.
 15. Theprimer composition according to claim 8 which is totally free oressentially free of particles of crosslinked nonaqueous dispersionresins and/or crosslinked microgel resins.
 16. The primer compositionaccording to claim 8 having a total solids concentration of at least40%.
 17. The primer composition of claim 8 or 15, wherein saidcomposition is a primer-surfacer beneath a composite basecoat/clearcoatfinish.
 18. A substrate coated with a dried and cured layer of thecomposition of claim 8 or
 15. 19. The substrate of claim 18 in which thesubstrate is a vehicle body or part thereof.
 20. A method for obtainingnormal film builds on an automotive substrate using a 3-layer wet paintsystem without a primer bake, which method comprises, (a) applying alayer of the primer composition of claim 8 to a substrate; (b) applyinga layer of a base coating composition wet-on-wet over said layer ofprimer composition; (c) applying a layer of a clearcoat compositionwet-on-wet over said layer of base coating composition; (d) curing theapplied three weight layers together in a single bake.